The invention relates to a process for the continuous production of methylmercaptan by reacting a starting gas mixture of methanol and hydrogen sulphide in the gas phase at a reaction temperature of between 200 and 600° C. and an operating pressure of 1.5 to 40 bar on a catalyst in a multi-bed reactor.
Methylmercaptan is an industrially important intermediate for the synthesis of methionine and for the production of dimethyl sulphoxide and dimethyl sulphone. Methylmercaptan is mainly produced from methanol and hydrogen sulphide by reaction on a catalyst consisting of an alumina support and transition metal oxides and basic promoters. The synthesis of the mercaptan is usually carried out in the gas phase at temperatures between 300 and 500° C. and at pressures between 1 and 25 bar. The reaction of hydrogen sulphide and methanol to give methylmercaptan is an exothermic process. DE-C 196 54 515 describes, for example, a process for the production of methylmercaptan in a tube bundle reactor, in which the liberated heat of reaction is dissipated by means of a salt melt and is then utilized indirectly by means of heat exchangers for the evaporation of methanol.
In addition to the methylmercaptan formed and water, the product gas mixture contains the unreacted starting substances methanol and hydrogen sulphide and, as by-products, dimethyl sulphide and dimethyl ether, and in small amounts also polysulphides (dimethyl disulphide). In accordance with the reaction, inert gases such as, for example, carbon monoxide, carbon dioxide, nitrogen and hydrogen are also present in the product gas.
The methylmercaptan formed, as explained in DE 17 68 826, is separated off from the product gas mixture in a number of distillation and scrubbing columns at temperatures between 10 and 140° C.
The reaction according to GB 14 17 532 can be carried out in a fixed bed reactor containing a number of catalyst beds or in a number of consecutive reactors. Methylmercaptan is prepared here by the reaction of a mixture of methanol with hydrogen sulphide in a molar ratio of 1.10:1 and 2.5:1, both reaction components being fed to the reactor separately. According to FR 24 77 538, for the production of methylmercaptan fresh hydrogen sulphide gas is compressed to 11 bar in a compressor. Afterwards, cycle gas led back from the process, which contains hydrogen sulphide, dimethyl sulphide, methanol and small amounts of methylmercaptan, is mixed with the compressed hydrogen sulphide for the formation of the starting gas mixture and this is heated to 510° C. Before entry into the first of up to 10 reactors connected in series, the washing agent cycle stream, which contains methanol and dimethyl sulphide, is admixed to the starting gas mixture, whereby the reaction entry temperature falls to 450° C. Before the second and the following reactors, further methanol, partly as a liquid and partly as a gas, is injected into the gas stream.
DE-C 11 34 368 relates to the use of a tube bundle reactor for the production of methylmercaptan. The reactor consists of a cylindrical container, in which the catalyst tubes are arranged parallel to one another. The catalyst tubes are welded below and above with tube cover plates, as in tube bundle heat exchangers, the intermediate spaces between the tubes being filled with heat-conducting fluid. Each catalyst tubes is provided at its lower end with a screen, which carries the particulate catalyst.
DE 196 54 515 relates to a process for the production of methylmercaptan, in which the energy needed for the evaporation of the methanol is partly introduced by utilization of the heat of compression of the hydrogen sulphide gas and by the heat content of the product gas leaving the reactor. The heat of reaction is utilized here in order to heat the starting gas mixture to the reaction temperature with the aid of an external gas heater.
The economy of the overall process depends crucially on the reaction of the starting gas mixture in a suitable pressure reactor and the preparation of this gas mixture. For example, large electrical powers are needed for the operation of the compressors and of the heating and cooling circuits. Further, expensive changes of catalyst in tube bundle reactors on account of long stoppage times represent a time and cost factor which is not to be neglected.
The object of the invention is the provision of an economic process for the production of methylmercaptan.